Isthmomys
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| Isthmomys Fossil range: Recent |
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Isthmomys is a genus of rodent in the Cricetidae family.
The yellow deer mouse (Isthmomys flavidus) and the Mt. Pirre deer mouse (I. pirrensis) are neotomine-peromyscine Panama endemics that have been recorded from disjunct localities of montane cloud forests at elevations ranging from 900-1500m. Although Panama has been the subject of numerous faunal surveys and ecological studies (Aldrich and Bole 1937; Anthony 1916; Bangs 1902; Fleming 1970; Goldman 1912, 1920; Handley 1966; Mendez 1993; Wenzel and Tipton 1966), museum records indicate that there appears to be no suitable habitats for members of this genus in the low-lying intermediate elevations. Thus, the occurrence of this genus and its population ranges have a propensity to be highly localized and dependent on unknown ecological factors.
Isthmomys flavidus, described by Bangs (1902), was discovered by W. W. Brown, Jr. on the southern slope of Volcan de Chiriqui (8° 49' N, 82° 32' W). He found it common in the upland forest from 1000 to 1500m, but no specimens were taken above or below these elevations (Bangs 1902; Goldman 1920; Goodwin 1946). Museum records specify two isolated populations in western Panama, one at Cerro Colorado where R. Pine et al. collected in 1980 (8° 31' 60N, 81° 49' 0W) and at Cerro Hoya on the Azuero Peninsula by C. Handley in 1962 (7° 23' N, 80° 38' W). The presence of I. flavidus or a closely allied form in Costa Rica is probable (Goodwin 1946), however, no specimens have been reported. There are no currently known fossil records of Isthmomys (McKenna and Bell 1997)
Isthomys pirrensis was described by Goldman (1912) and at the time it was considered as the “most common rat of southeastern Panama” (Anthony 1916). Found only in the eastern region of Darien, I. pirrensis appears to be a relic species confined to the Panamanian side of the Serrania del Darien (Hershkovitz 1966). Its occurrence on the Colombian side of the Serrania has been suggested without documentation of specimens (Alberico et al. 2000; Cuartas-Calle and Muñoz-Arango 2003; Hall and Kelson 1958; Hershkovitz 1966). Localities for I. pirrensis indicated by museum records are in the eastern Panama state of Darien throughout the areas surrounding Mt. Pirre (=Pirri, 7° 51' 8 N, 77° 46' 24 W), Mt. Tacarcuna and the Tacarcuna Village (8° 10' 58 N, 77° 17' 3.5 W); however, no specimens are recorded from the low lying areas (<800m) that bisect these two ranges. Although neotomine-peromyscine rodents have historically been the subject of systematic research, studies involving Isthmomys have generally excluded I. flavidus due to its rarity in museum collections and the unavailability of tissue samples for molecular analysis (Bradley et al. 2007; Bradley et al. 2004; Carleton 1973; Engel et al. 1998; Hill 1975; Stangl and Baker 1984). In addition, morphological and physiological studies involving I. flavidus (Carleton 1973, 1980; Hill 1975; Hooper and Musser 1964; Linzey and Layne 1974; Linzey and Layne 1969; Voss and Linzey 1981) utilized specimens from the Cerro Hoya locality which was originally proposed as a geographic race of I. pirrensis (Handley 1966).
Middleton (2007) evaluated geographic and non-geographic variation within the genus and defined species limits considering the isolated nature of known populations. A few cranio-dental traits have been used to discriminate the two species. Most notable is the tendency of a 6-tuberculate condition in the first upper molar of I. flavidus (vs. the 5-tuberculate condition in I. pirrensis) (Goldman 1920) and the presence of a subsquamosal fenestra in I. pirrensis which generally is absent in I. flavidus (Carleton 1980, 1989). It is important to note that the inclusion of Isthmomys in the Reithrodontomyini tribe is not definite. Molecular data for this genus is available for I. pirrensis only, and sequences from different genes have placed this species at two opposite poles within the Peromyscini (either with Reithrodontomys or Baiomys). Insufficient genetic sampling of the entire distribution of the genus therefore limits the ability to properly identify the taxonomy not only of the genus, but within the genus as well.
[edit] References
- Anthony, H. E. 1916. Panama Mammals Collected in 1914-1915. Bulletin of the American Museum of Natural History 35: 357-377.
- Bangs, Outram. 1902. Chiriqui Mammalia: Megadontomys flavidus. Bulletin of the Museum of Comparative Zoology at Harvard College 8: 27-29.
- Carleton, M. D. 1980. Phylogenetic relationships in neotomine-peromyscine rodents (Muroidea) and a reappraisal of the dichotomy within New World Cricetinae.
- Cuartas-Calle, Carlos Auturo and Javier Muñoz-Arango. 2003. Lista de los Mamiferos (Mammalia: Theria) del departamento de Antioquia, Colombia. Biota Colombiana 4, no. 1: 65-78.
- Goldman, Edward. 1912. New Mammals from Eastern Panama. Smithsonian Miscellaneous Collections 60, no. 2: 1-18.
- Goldman, Edward. 1920. Mammals of Panama. Smithsonian Miscellaneous Collections 69, no. 5.
- Goodwin, George G. 1946. Mammals of Costa Rica. Bulletin of the American Museum of Natural History 87, no. 5.
- Hall, E.R. and K.R. Kelson. 1958 The Mammals of North America. New York: Ronald Press Co.*Handley, Charles. 1966. Checklist of the Mammals of Panama. In Ectoparasites of Panama, ed. Wenzel and Tipton:753-796. Chicago, Illinois: Field Museum of Natural History.
- Hooper, Emmet T. and Guy Musser. 1964. Notes on the classification of the rodent genus Peromyscus. Occasional Papers of the Museum of Zoology, University of Michigan, no. 635.
- Linzey, Alicia and James N. Layne. 1974. Comparative morphology of spermatozoa of the rodent genus Peromyscus (Muridae). American Museum Novitates, no. 2532.
- Linzey, Alicia and James N. Layne. 1969. Comparative morphology of the male reproductive tract in the rodent genus Peromyscus (Muridae). American Museum Novitates, no. 2355: 1-46.
- Middleton, John R. 2007. A Systematic Revision of Genus Isthmomys (Rodentia: Cricetidae). Masters Thesis, Texas Tech University, Lubbock Texas.
- Musser, G. G. and M. D. Carleton. 2005. Superfamily Muroidea. Pp. 894-1531 in Mammal Species of the World a Taxonomic and Geographic Reference. D. E. Wilson and D. M. Reeder eds. Johns Hopkins University Press, Baltimore.
- Stangl, F. B. and R. J. Baker. 1984. Evolutionary relationships in Peromyscus: congruence in chromosomal, genic, and classical data sets. Journal of Mammalogy 65, no. 4: 643-654.
